Does anyone know what the signal from the ASM to the shocks is for switching the CATS on/off? I mean the two-wires running into each shock.

As far as I can tell, the shocks have a "soft" setting for comfort ride or a "firm" setting for sports ride. It's either on or off.

So i'd guess that it's just a voltage which actuates a solenoid inside the shock. I'm sure someone has disassembled a dead shock, what did you see in there?

The wiring diagrams don't give any clue as to what the signal is, just says output + and -.

From the XJ Tech Guide it also mentions that in the event of an Air Suspension Fault, the shocks default to "firm".

To me that suggests that the actuated state (with voltage) must be "soft" and non-actuated (without voltage) must be "firm", so if the ASM couldn't give those outputs, the shocks default to firm.

So to bypass the CATS operation we need to isolate the shock from the ASM, but at the same time keeping the circuit to the ASM closed (it throws codes if open-circuit), and introducing a permant voltage to the shock to keep it "soft", how does that sound?

Why am I asking this?

I want to take the XJR down the 1/4 mile. But the CATS stiffens up the rear of the car at launch which hurts traction. If we can trick the shocks into staying "soft" then this will help get the car off the line.

Anyone able to help?

 

Right well i'm really not sure what to make of the measurements;
I tested the left front shock since it's the easiest one for me to access.

Disconnected the plug to the top of the shock, with the ignition off I got;

At the plug itself from the ASM;
-6.45MOhm (yes minus ohms???) and 0Ohms when reversed, huh?
DC voltage with Red/white as Pos, Yellow/Black as Neg
Which bounced around rythmically 10.5V up to 11.V, it was really a steady pulse.

Measured the resistance of the pins on the shock itself at 0.6 Ohms.

That seems like some kind of serial or digital signal, pity I don't have an oscilliscope to see what these pulses look like.

I tried a bump test on the left corner with the engine running; got it to really squat down with pretty much my full body weight on it, but only briefly, and only once, after that it was very stiff...

BTW with the plug out and the multimeter on it I got CATS SYSTEM FAILURE on the dash, went away immediately after plugging back in. Checked the code after "open circuit left front shock".

I'll be surprised if it really is a digital signal, I mean Jag documents refer to an "actuator" and i'm pretty sure the only "actuator" that'll run on 2-wires is a solenoid coil.

I didn't want to strip back the insulation on the wires running to the shock (hello corrosion & shorts!), but seems that's the next step to try & get a measurement with the CATS functioning and the engine running...

Or just try putting a 0.6 Ohm resistor in there...?

Supposing that a resistor will keep the errors at bay, then I still have to figure out what to send to the shock, obviously it's not a simple 12V. The only way to figure that out is a measurement with the engine running and without triggering the "firm mode"...

Anyone out there actually interested in this?

 

Not exactly what you are after but may help complete some of the picture:

The damper operation and design are carry over, each unit having a 400Hz Pulse Width Modulated, two stage (open / closed) 5.4 ohm solenoid.
...
ASCM operating voltage is between 7-20 volts and has reverse voltage protection.
...
The vertical accelerometer signal input to the ADCM is from two Texas Instruments accelerometers, in the form of a variable voltage between 0.25 (high 'g') - 4.75 (low 'g').

The vertical accelerometers receive a 5v supply from the ASCM and the voltage output is relative to the vehicle g' force.

Within each accelerometer an internal contact closes under pressure by the vehicles inertia forces, the pressure acting on the contacts increases the electrical capacitance in the circuit and subsequently reduces voltage output to the ADCM.

The changing voltage to the ADCM from the accelerometers decides the strategy of the damper switching, which now has enhanced functionality.
...
Longitudinal
The front and rear switching is to assist in resisting anti-dive and anti-squat characteristics as the vehicle accelerates and subsequently brakes.

Inputs required to provoke this switching action are: brake switch, brake pressure (Longitudinal decceleration) and engine torque rate (Longitudinal acceleration) via the CAN bus with the appropriate vehicle speed message.

 

Thank-you! Thank-you! Thank-you! Yes it's the "anti-squat" that I want to defeat.

Was there any other info in your source about the height automatically adjusting during the anti-squat characteristics?

I could imagine that even after bypassing the CATS that the rear suspension would increase pressure to hold the target ride height, which also acts as an anti-squat function....

But we're getting somewhere now. So a 5.4Ohm resistor that can handle 65watts might do the trick of keeping the error messages at bay.

Now it's just a matter of reproducing this 400Hz PWM signal in a way that keeps the shock in soft mode.

I had a different idea of PWM from my old days as an apprentice, to me that was only used as a proportional signal, certainly not on or off.

I had to look up how it can relate to on/off solenoids http://www.axiomatic.com/pulse-width-modulation.pdf so basically the PWM is a way of introducing a "constant" voltage/current while reducing the heat in the solenoid = longer lifetime.

If that's the case with the shock then it's probably not a good idea to put a flat DC voltage onto it....

But it does make sense that the measured voltage was a constant, rythmic pulse. 400Hz would explain the indications. My little meter doesn't measure square-wave frequency, only AC...and in any case it's the width of the pulse that's interesting now.

So i'll have to borrow a scope with datalogger from somewhere & have a look at how the signal changes; cruising in soft mode vs. a launch that triggers firm mode.

Thanks again.

 

If you mean the air springs, then adjustment is inhibited:

Levelling
The system will not attempt to level whilst braking, accelerating and cornering.

Signals are:

• Braking - Pressure Transducer and Switch
• Cornering - Lateral Accelerometer (Teves)
• Acceleration - Engine Torque Signal

 

Hmmm....so in that case we should be good!

I've borrowed a Fluke ScopeMeter from work, hope I can get some measurements of the signal to the rear shocks & flush out what would be needed to lock them in the soft mode.

 

OK so I got the ScopeMeter hooked up to one of the rear shocks.

With the engine off it's actually 0V, flat, dead nothing.

Engine running it's the same, unless there is a height adjustment, when you get this;



Let's call it 5 Volts, a square wave pulse, 400Hz, with a 1ms pulse width.

While driving, you also have the exact same signal to the shock.

When you come to a complete stop, it goes back to 0V.

Under heavy braking, it also goes to 0V.

What I couldn't really determine, is how the signal works at launch.

As soon as the car moves, the pulses start.

I tried a launch in Sport mode and it seemed to have a little delay.

A launch not in Sport mode saw the pulses start immediately.

So, it seems that soft mode is this 5V, 400Hz, 1ms pulse width signal, and firm mode is 0V.

If locking the rear shocks in soft mode will make much of a difference to a launch remains to be seen. But at least we know how to do it now.

 

Talk about determination !!!!!!!!!!!!!! I like it.

 

So if I want firmer shocks and don't care about warning codes I can just unplug them? I have to say the Buick like ride is one think I am not so thrilled about. Of course the lotus with the non adjustable sport suspension rattles my teeth out and the defender with heavy duty suspension and bilstien shocks is not much better than the lotus.

 

You could simply terminate the connections by putting some resistors in the plugs, at least 5.4Ohms and capable of 65watts. I know of at least one S-Type where the resistor trick has worked, he used 10Ohm resistors. This stopped the error messages.

 

Hello,
Is there any way you can show/explain how he connected the resistors in the event I want to do it (trying to verify my understanding)? I do NOT want to create another issue by attempting to remedy one incorrectly. Thanks!

 

The connection to the solenoid in the shock is just 2 pins. You would just close the circuit using a resistor.

Here's a pic of a similar thing using a paperclip, instead of a paperclip, use a resistor.

 

Cambo351

Did you find the solution to supply shocks with required feed?

 

No, to be honest I gave up on it.

My train of thought at the time was that the air suspension was not allowing the rear of the car to "squat" while launching on the drag strip.

This picture changed my mind about that.